If you read the article I gave you (Lumolabs), the error made in between focus sensors are expected to be

My left AF sensor can be improved, and I could scream about backfocusing issue. But the truth is one of the other D800 I have tested as the same, and another one was a bit worse. The lumolabs article shows that a D3 they tested was like my D800.

So, if I want it to be a little bit better, I'll ask Nikon to service it before the end of the warranty period. For now, I am fine with it. With real life examples, it is fine enough.